Generally, as methods for crystallization of an organic matter, there are (1) Melting-type Purification, (2) Crystallization Using Solvent (being soluble each other), (3) Cooling Crystallization with Solution being Insoluble in Organic Matter (such as water).
On the other hand, cooling methods for industrial crystallization can be classified as follows.
(A) Indirect Cooling: A molten organic matter is fed into a thin, cylindrical vessel. A cooling jacket is provided on the outside of the vessel and cooling water is passed through the cooling jacket. Thus, crystals are formed and attached to the inner surface of the above vessel. These crystals are scraped and deposited with means such as a scraping blade so as to be withdrawn in the form of slurry and separated outside of this system.
(B) Direct Cooling: A molten organic matter is fed into a vessel having an agitator. At the same time, a solvent being insoluble in the organic matter, such as water, is fed to the vessel. Thus, cooling is carried out for crystallization, while the organic matter is dispersed in the water.
(C) Adiabatic Cooling: An organic matter is dissolved in a solvent and operated under the condition of self-evaporation performed in a crystallizer. Thus, a produced vapor is compressed by a compressor and condensed by a condenser. In this process, a condensed refrigerant is recycled to the crystallizer.
The above method (1) Melting-Type Purification has a fundamental defect that the organic matter is deteriorated because of high temperature at which the treated organic matter is melted.
Further, the method (A) Indirect Cooling, due to the low heat transfer coefficient of the order of 200-500 Kcal/m.sup.2. .degree.C. hr, requires a large sized apparatus and a high construction cost.
In the method (B) Direct Cooling classified in the above system (3), small quantity of the organic matter is often dissolved in the water. Thus, its treatment requires a high cost.
On the other hand, in the method (2), that is adiabatic self-evaporation cooling using the solvent, a crystallizer should be operated at a condensing temperature which is too low for normal cooling water to condense. Accordingly, large sized refrigerating equipment is required resulting in a high construction cost. Further, when the method (C) Adiabatic Cooling is adapted, in general, a compressor should be used in order to rise a compressing temperature under a high pressure. Accordingly, a high operation cost is required for the compressor. In some cases, depending on an evaporating temperature and a temperature of cooling water, the compressor is not required. However, since the condensing temperature of the refrigerant is usually under 0.degree. C., even if the compressor is not used, brine must be produced by a refrigerator for condensation, which causes a heavy investment for a construction cost related to refrigerating equipment.